In carrying out various oil well completion and production operations, after a borehole has been formed several thousand feet down into the ground, a casing must be cemented into place in order to separate the various different fluid producing stratus of the earth from one another and also to prevent various different formation fluids from mixing with one another. After the casing has been cemented into place, the lower casing string is perforated adjacent a hydrocarbon producing formation so that the well can be produced. Where the pay zone is inadequate to sustain the cost of production, the well is plugged and abandoned thereby losing the valuable casing. Accordingly, it would be desirable to be able to retrieve part of the casing from the borehole, thereby enabling the casing to be subsequently used in another borehole and effecting a savings of thousands of dollars.
During the cementing operation, it is desirable that cement be pumped downhole through the entire casing string and back uphole through the borehole annulus, thereby assuring that the outer periphery of the casing is bonded along its entire length to the borehole wall. Accordingly, it is desirable that the cementing take place in two different stages thereby avoiding the difficulties involved with forcing cement several thousand feet down through the casing and several thousand feet back up the borehole annulus. It would further be desirable that staging be carried out by first pumping cement down the entire casing string and back up the borehole annulus toward a medial portion of the string, and thereafter the cement pumped only halfway down the casing and laterally into the annulus after which the cement is again forced back up the upper borehole annulus to the surface of the ground.
It would especially be desirable to be able to carry out the cementing operation without the necessity of drilling cement from the interior of the casing string.